Digital continuous potentiometer servo feedback element

ABSTRACT

A servo system feedback element including a potentiometer in association with commutating and logic circuitry for providing a continuous and non-ambiguous saw-tooth follow-up output.

United States Patent Cohen et al.

1131 3,662,246 1451 May 9,1972

[72] inventors: Richard Cohen, Old Bridge; Harold Morelnes, Springfield; Walter Parfomak, Wallington, all of NJ. 4

[73] Assignec: The Bendix Corporation [22] Filed: Mar. 25, 1971 [21] Appl. No.: 127,908

' [52] U.S. Cl ..3l8/663,3l8/590 [5i] lnt.CI. [58] FieIdofSearch ..G05b 1/06 3 l 8/663, 602, 664, 665, S99

I [56] I 11mm cm UNITED STATES PATENTS 2,944,021 7/l96l Hinz ..3l8/663 3,l55,889 ll/l964 Stiles et al .3 l8/602 3,239,736 3/1966 Gardberg ..3 l8/602 x Primary Examiner-Benjamin Dobeck Attorney-Anthony F. Cuoco and Flame, Hartz. Smith &

Thompson ABSTRACT A servo system feedback element including a potentiometer in association with commutating and logic circuitry for providing a continuous and non-ambiguous saw-tooth follow-up output.

9 Chins, 2 Drawing Figures OUTPUT CIRCUIT) ISOLATION 4 SIGNAL DIRECTING 1 PRE AMP. '"l a AMPLIFIER cmcurr CIRCUIT I CIRCUIT 1 I COMMAND I )4I SIGNAL I 70 I 1 1 1 I 1 1 I I i 76 I L l EXCLUSIVE 0R 7 GATE cmcurr 4s 1 DIGITAL CONTINUOUS POTENTIOMETER SERVO FEEDBACK ELEMENT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to feedback elements for servo systems and, particularly, to potentiometer type feedback elements. Moreparticularly, the invention relates to a digital, continuous potentiometer servo feedback element.

2. Description of the Prior Art Servo systems used in aircraft instrumentation are of the digital, continuous closed loop type including a feedback element in the servo loop. Since synchros'have the characteristic. of 360 angular displacement and thereby provide a continuous output without deadband, they have heretofore been universally used as the feedback element. Conventional potentiometers have limited angular displacement (about 350) and because of this are not suitable for the purposes described.

The continuous potentiometer of the invention overcomes this difficulty and has several additional advantages.- Included among these are that simple digital to analog converters rather than more complicated digital to synchro converters may be used in the system and discrete most significant bit (M58) inputs from digital signals may be applied. Further, an all d.c. feedback loop is thus provided.

SUMMARY OF THE INVENTION This invention contemplates a potentiometer in cooperative arrangement with a commutating switch and associated logic circuitry. The commutating switch is open for -l 80 and closed for 180 360 displacement of the potentiometer wiper arm. When the wiper arm is in the 0-1 80 region, logic circuitry is effective for grounding one potentiometer tap or terminal and for rendering the other tap positive; and when the arm is in the 180 360 (0) region, the one tap is positive and the other tap is grounded.

When the wiper arm is displaced in a clockwise direction the potentiometer output to ground increases linearly from zero volts at 0 to maximum volts at 180, after which it instantaneously returns to zero and increases linearly to maximum volts at 360 for providing a continuous saw-tooth output.

The main object of this invention is to provide a digital, continuous potentiometer servo feedback element.

Another object of this invention'is to provide a potentiometer feedback element of the type described which enables simple digital to analog converters to be used and discrete most significant bit inputs from digital signals to be applied.

Another object of this invention is to provide an all d.c. servo feedbackloop. v

Another object of this invention is to provide a potentiometer in cooperative arrangement with commutating and logic circuitry for providing a continuous and non-ambiguous sawtooth follow-up output.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for illustration purposes only and is not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical schematic diagram of a potentiometer feedback element according to the invention.

FIG. 2 is a graphic representation of the potentiometer output signal provided according to the invention.

DESCRIPTION OF THE INVENTION With reference to FIG. I, a d.c. command signal 5,. provided by a command signal means 2 is applied to an isolation 2 amplifiercircuit designated generally by the numeral 4. Amplifier circuit 4 includes an operational amplifier 6 having an input terminal connected to command signal means 2 and another grounded input terminal. A feedback loop connected to an output terminal of the amplifier and to the grounded input terminal includes a resistor 8 and a variable resistor 10.

The output from isolation amplifier circuit 4 is applied to a signal directing circuit designated generally by the numeral 12. Signal directing circuit 12 includes a resistor 14 connected to the output terminal of amplifier 6 in isolation amplifier circuit 4 and a resistor'l6 serially connected to resistor 14 and grounded through a capacitor 24.

A resistor 18 is connected to an arm 20A of a potentiometer 20 and serially connected to a resistor 22 which is grounded through capacitor 24. A resistor 26 is connected to the output terminalof amplifier 6 and is serially connected to a resistor 28. Resistor 28 is grounded through a resistor 30 and a capacitor 34. I

A resistor 36 is connected to arm 24A of potentiometer 20 and is serially connected to a resistor 40 which is grounded through resistor 30 and capacitor 34. v

A transistor 06 in signal directing circuit 12 has a grounded emitter element and a collector element connected intermediate resistors 14 and 16. Transistor Q6 has a base element connected to a suitable source of positive direct current shown as a battery 38 through resistors 40 and 42.

A transistor Q7 has a grounded emitter element and a collector element connected intermediate resistors 36 and 40. Transistor Q7 has a base element connected through a resistor 46 to an exclusive OR gate circuit designated generally by the numeral 45.

A transistor O8 in signal directing circuit 12 has a grounded emitter element and a collector element connected intermediate resistors 26 and 28. A transistor 09 has agrounded emitter element and a collector element connected intermediate resistors 18 and 22. The base elements of resistors Q8 and Q9 are connected to a point 50 in signal directing circuit 12 through resistors 52 and 54, respectively.

The output from signal directing circuit 12 at a point 12A is applied to a pre-amp circuit designated generally by the numeral 60. Pre-amp circuit 60 includes an operational amplifier 62 having an input terminal connected to point 12A in signal directing circuit 12 and another input terminal connected to ground through resistor 30 and capacitor 34. A feedback loop is connected to the grounded input terminal and to an output terminal of amplifier 62, and which feedback loop includes a resistor 64.

The output of pre-amp circuit 60 at a point 60A is applied to an output circuit designated generally by the numeral and including a transistor 01! and a transistor Q12. The base elements of transistors Q11 and 012 are connected to point 60A through a resistor 70 and the emitter elements of the transistors are connected to a d.c. torquer 72. Transistor 012 has a collector element connected to a suitable source of positive direct current shown as a battery 74 and transistor Q]! has a collector element connected to a suitable source of negative direct current shown as a battery 76.

DC. torquer 72 is connected through suitable mechanical means 77 and 79 to arm 20A of potentiometer 20 for angularly displacing the arm. Torquer 72 is connected through suitable mechanical means 77 and 78 to a commutating switch I 208 is connected to a collector element of a transistor Q1 and which transistor 01 has a grounded emitter element and a base element connected to battery 38 through a resistor 84.

The base element of transistor O1 is connected to the collector element of a transistor Q2. Transistor Q2 has a grounded emitter element and a base element connected through a resistor 86 to switch S1 and connected through resistor 86 and a resistor 88 to a base element of a transistor 03. Transistor Q3 has a grounded emitter element and a collector element connected to potentiometer tap 20C. Tap 20C is connected through a resistor 90 and a variable resistor 92 to battery 38. Transistors Q1, Q2 and Q3 provide logic circuitry having a purpose to be hereinafter described, and which circuitry cooperates with potentiometer 20 to provide a potentiometer and logic network designated generally by the numeral 5.

Exclusive OR gate 45 includes a transistor Q10 having a collector element connected to the base element of transistor 46 in signal directing circuit 12 through resistor 46 and a grounded emitter element. Transistor Q10 has a base element connected through resistor 100, a diode 102, a resistor 104, a diode 110 and a resistor 112 to a digital signal means 3 which provides a signal E,,,,,, corresponding to the most significant bit of a digital servo system output signal.

A transistor Q4 has a collector element connected intermediate diode 102 and resistor 104, and a transistor Q5 has a collector element connected intermediate resistor 90 and a diode 92, and which diode is connected to resistor 90 and connected to point 50 in signal directing circuit 12. The emitter elements of transistors Q4 and Q5 are grounded and the base element of transistor O4 is connected through a resistor 106 and resistor 86 to the base element of transistor Q2 in potentiometer and logic circuit 5. Transistor Q5 has a base element connected through a resistor 108 and through diode 110 and resistor 112 to digital signal means 3.

Digital signal means 3 is connected to switch S1 through resistor 112 and through a circuit including a diode 111 serially connected to resistor 112, a resistor 115 connected to battery 38 and a diode 114 serially connected to a grounded zener diode 116. Diode 111 is connected intermediate diodes 114 and 116 and switch S1 is connected through diode 118 to a point 120 intermediate diode 114 and resistor 115.

OPERATION Switch S1 is open in the i80 region and closed in the 180 360 region of travel of arm 20A of potentiometer 20. When switch S1 is open, transistors Q1 and Q2 are conductive and transistor 03 is non-conductive. When the transistors are in this state, tap 20B of potentiometer 20 is grounded and tap 20C is positive When switch S1 is closed, the voltage applied through the switch reverses the state of the transistors so that potentiometer terminal 20C is grounded and terminal 20B is positive.

When potentiometer arm 20A rotates in a clockwise direction the output from the potentiometer arm to ground linearly increases from zero volts at 0 to maximum d.c. volts (V,,,,,,.) at 180. Past 180", the output instantaneously drops to zero and then again linearly rises to maximum d.c. volts at 360 to provide a saw-tooth waveform as shown in FIG. 2. Variable resistors 82 and 92 are used for potentiometer excitation voltage adjustment.

Exclusive OR gate circuit 45 and signal directing circuit 12 cooperate to insure the selection by the servo loop of the shortest path to its command position. This is accomplished by sensing the follow-up potentiometer arm position with respect to the command signal and, in turn, controlling the servo error polarity which the controls servo direction.

Output signal E,,,,,, from digital signal means 3 corresponds to the position of the command function. For the 0 to 180 region, signal E is zero and for the 180 to 360 region, signal E,,,,,, is high". The position of potentiometer arm 20 is established by the state of commutating switch S1, as is signal E,,,,,,. The output from switch S1 is zero" in the 0 to 180 region and is high" in the l80 360 region. A relative output from digital signal means 3 and commutating switch S1 is utilized for providing proper servo direction.

For equal inputs from digital signal means 3 (signal E,,,,,,) and switch S1 to transistors 04 and O in exclusive OR gate circuit 45, the voltage output, directed to the bases of transistors Q8, Q9 and Q10 is zero. Transistors Q6 and 07 remain conductive while transistors Q8 and 09 are non-conductive, thus allowing the error signal to reach pre-amp circuit '60 undisturbed. If the outputs from digital signal means 3 and switch S1 differ, the output from gating circuit 45 becomes high, thus reversing the state of transistors 06 and Q7 and transistors Q8 and Q9 and causing the change in polarity of the servo error signal. A correct servo direction then follows.

Since pre-amp circuit 60 can not directly drive torquer 72, a complementary pair of power transistors Q11 and Q12 arranged as emitter followers is used.

Although but a single embodiment of the invention is illustrated and described in detail, it is to be expressly understood that the invention is not limited thereto. Various changes may also be made in the spirit and scope of the invention as the same will now be understood by those skilled in the art.

What is claimed is:

1. A digital, continuous servo system feedback device comprising:

potentiometer means including a displaceable arm and providing an output signal in accordance with the arm displacement;

logic circuitry connected to the potentiometer;

commutating switch means connected to the logic circuitry;

means for providing a servo system command signal;

means for providing a digital servo system output signal;

means connected to both of said signal providing means and coupled to the potentiometer arm and to the switch means, and responsive to the command signal and to the digital output signal for displacing the arm and for actuating the switch means in accordance with said displacement; and

said switch means being effective upon actuation thereof for operating the logic circuitry to affect the potentiometer output signal so that said signal has a continuous and nonambiguous saw-tooth wave form.

2. A system as described by claim 1, wherein:

the potentiometer has first and second output terminals;

the potentiometer arm'is angularly displaceable through a range of 360;

the logic circuitry includes first current flow control means connected to the first potentiometer output terminal and second current flow control means connected to the second potentiometer output terminal; and

the potentiometer arm being displaced in a predetermined sense and the switch means actuated so that for 0 to 1 arm displacement, the switch means is open to render the first current flow control device conductive and the second current flow device non-conductive, and for to 360 arm displacement, the switch means is closed to render the first current fiow device non-conductive and the second current flow device conductive.

3. A system as described by claim 2, wherein:

when the first current flow control device is conductive and the second current flow control device is non-conductive, the first potentiometer output terminal is grounded and a signal at a predetermined level in one sense is provided at the second potentiometer output terminal; and

when the second current flow device is conductive and the first current flow control device is non-conductive, the second potentiometer output terminal is grounded and the signal at the predetermined level in the one sense is provided at the first potentiometer output terminal.

4. A system as described by claim 2, wherein:

the potentiometer output signal linearly increases from zero voltage at 0 to maximum voltage at 180 potentiometer arm displacement; and

past 180 potentiometer arm displacement the potentiometer output signal instantaneously drops to zero and linearly increases to maximum at 360 arm displacement to provide the continuous and non-ambiguous saw-tooth wave form.

5. A system as described by claim 2, wherein the means connected to both of said signal providing means and coupled to the potentiometer arm and the switch means, and responsive to the command signal and the digital output signal for displacing the arm and for actuating the switch means in accordance with the displacement includes:

-the digital signal corresponding to the displacement of the potentiometer arm;

the switch means providing a signal corresponding to one of its open and closed states;

circuit means connected to the digital signal means, the

command signal means and the switch means and responsive to the signals therefrom for providing a direction control signal; and

means connected to the direction control signal means for actuating the switch and for displacing the potentiometer arm in response to the signal therefrom.

6. A system as described by claim 5, wherein the circuit means for providing a direction control signal includes:

means connected to the digital signal means and to the switch means for comparing the signals therefrom and for providing a signal in accordance with said comparison; and

means connected to the comparing means and to the command signal means and responsive to the signals therefrom for providing the direction control signal.

7. A system as described by claim 6, wherein the means connected to the digital signal means and to the switch means for comparing the signals therefrom and for providing a signal in accordance with said comparison includes:

gating means for providing a signal at one level when the digital and switch means signals are equal and for providing a signal at another level when said signals differ.

8. A system as described by claim 7, further including:

a signal directing circuit having first and second current flow control means responsive to the signal at the one level from the gating means so that said first current flow control means is conductive and the second current flow control means is non-conductive, and responsive to the signal at the other level from the gating means so that the second current flow control means is conductive and the first current flow control means is non-conductive; and said signal directing circuit being responsive to the command signal when the first current flow control means is conductive for providing a direction control signal in one sense, and responsive to said command signal when said second current flow control device is conductive for providing a direction control signal in the opposite sense.

9. A system as described by claim 5, wherein the last mentioned means includes:

a pre-amp circuit:

an emitter follower circuit connected to the pre-amp circuit; and

a torquer connected to the emitter follower circuit and energized thereby for actuating the switch and for dis-' placing the potentiometer arm.

i l l 

1. A digital, continuous servo system feedback device comprising: potentiometer means including a displaceable arm and providing an output signal in accordance with the arm displacement; logic circuitry connected to the potentiometer; commutating switch means connected to the logic circuitry; means for providing a servo system command signal; means for providing a digital servo system output signal; means connected to both of said signal providing means and coupled to the potentiometer arm and to the switch means, and responsive to the command signal and to the digital output signal for displacing the arm and for actuating the switch means in accordance with said displacement; and said switch means being effective upon actuation thereof for operating the logic circuitry to affect the potentiometer output signal so that said signal has a continuous and nonambiguous saw-tooth wave form.
 2. A system as described by claim 1, wherein: the potentiometer has first and second output terminals; the potentiometer arm is angularly displaceable through a range of 360*; the logic circuitry includes first current flow control means connected to the first potentiometer output terminal and second current flow control means connected to the second potentiometer output terminal; and the potentiometer arm being displaced in a predetermined sense and the switch means actuated so that for 0* to 180* arm displacement, the switch means is open to render the first current flow control device conductive and the second current flow device non-conductive, and for 180* to 360* arm displacement, the switch means is closed to render the first current flow device non-conductive and the second current flow device conductive.
 3. A system as described by claim 2, wherein: when the first current flow control device is conductive and the second current flow control device is non-conductive, the first potentiometer output terminal is grounded and a signal at a predetermined level in one sense is provided at the second potentiometer output terminal; and when the second current flow device is conductive and the first current flow control device is non-conductive, the second potentiometer output terminal is grounded and the signal at the predetermined level in the one sense is provided at the first potentiometer output terminal.
 4. A system as described by claim 2, wherein: the potentiometer output signal linearly increases from zero voltage at 0* to maximum voltage at 180* potentiometer arm displacement; and past 180* potentiometer arm displacement the potentiometer output signal instantaneously drops to zero and linearly increases to maximum at 360* arm displacement to provide the continuous and non-ambiguous saw-tooth wave form.
 5. A system as described by claim 2, wherein the means connected to both of said signal providing means and coupled to the potentiometer arm and the switch means, and responsive to the command signal and the digital output signal for displacing the arm and for actuating the switch means in accordance with the displacement includes: the digital signal corresponding to the displacement of the potentiometer arm; the switch means providing a signal corresponding to one of its open and closed states; circuit means connected to the digital signal means, the command signal means and the switch means and responsive to the signals therefrom for providing a direction control signal; and means connected to the direction control signal means for actuating the switch and for displacing the potentiometer arm in response to the signal therefrom.
 6. A system as described by claim 5, wherein the circuit means for providing a direction control signal includes: means connected to the digital signal means and to the switch means for comparing the signals therefrom and for providing a signal in accordance with said comparison; and means connected to the comparing means and to the command signal means and responsive to the signals therefrom for providing the direction control signal.
 7. A system as described by claim 6, wherein the means connected to the digital signal means and to the switch means for comparing the signals therefrom and for providing a signal in accordance with said comparison includes: gating means for providing a signal at one level when the digital and switch means signals are equal and for providing a signal at another level when said signals differ.
 8. A system as described by claim 7, further including: a signal directing circuit having first and second current flow control means responsive to the signal at the one level from the gating means so that said first current flow control means is conductive and the second current flow control means is non-conductive, and responsive to the signal at the other level from the gating means so that the second current flow control means is conductive and the first current flow control means is non-conductive; and said signal directing circuit being responsive to the command signal when the first current flow control means is conductive for providing a direction control signal in one sense, and responsive to said command signal when said second current flow control device is conductive for providing a direction control signal in the opposite sense.
 9. A system as described by claim 5, wherein the last mentioned means includes: a pre-amp circuit: an emitter follower circuit connected to the pre-amp circuit; and a torquer connected to the emitter follower circuit and energized thereby for actuating the switch and foR displacing the potentiometer arm. 